A plasma processing apparatus generates a plasma in a process chamber for treating a workpiece supported by a platen in the process chamber. A plasma processing apparatus may include, but not be limited to, doping systems, etching systems, and deposition systems. The plasma is generally a quasi-neutral collection of ions (usually having a positive charge) and electrons (having a negative charge). The plasma has an electric field of about 0 volts per centimeter in the bulk of the plasma. In some plasma processing apparatus, ions from the plasma are attracted towards a workpiece. In a plasma doping apparatus, ions may be attracted with sufficient energy to be implanted into the physical structure of the workpiece, e.g., a semiconductor substrate in one instance.
Turning to FIG. 1, a block diagram of one exemplary plasma doping apparatus 100 is illustrated. The plasma doping apparatus 100 includes a process chamber 102 defining an enclosed volume 103. A gas source 104 provides a primary dopant gas to the enclosed volume 103 of the process chamber 102 through the mass flow controller 106. A gas baffle 170 may be positioned in the process chamber 102 to deflect the flow of gas from the gas source 104. A pressure gauge 108 measures the pressure inside the process chamber 102. A vacuum pump 112 evacuates exhausts from the process chamber 102 through an exhaust port 110. An exhaust valve 114 controls the exhaust conductance through the exhaust port 110.
The plasma doping apparatus 100 may further includes a gas pressure controller 116 that is electrically connected to the mass flow controller 106, the pressure gauge 108, and the exhaust valve 114. The gas pressure controller 116 may be configured to maintain a desired pressure in the process chamber 102 by controlling either the exhaust conductance with the exhaust valve 114 or a process gas flow rate with the mass flow controller 106 in a feedback loop that is responsive to the pressure gauge 108.
The process chamber 102 may have a chamber top 118 that includes a first section 120 formed of a dielectric material that extends in a generally horizontal direction. The chamber top 118 also includes a second section 122 formed of a dielectric material that extends a height from the first section 120 in a generally vertical direction. The chamber top 118 further includes a lid 124 formed of an electrically and thermally conductive material that extends across the second section 122 in a horizontal direction.
The plasma doping apparatus further includes a source 101 configured to generate a plasma 140 within the process chamber 102. The source 101 may include a RF source 150 such as a power supply to supply RF power to either one or both of the planar antenna 126 and the helical antenna 146 to generate the plasma 140. The RF source 150 may be coupled to the antennas 126, 146 by an impedance matching network 152 that matches the output impedance of the RF source 150 to the impedance of the RF antennas 126, 146 in order to maximize the power transferred from the RF source 350 to the RF antennas 126, 146.
The plasma doping apparatus may also include a bias power supply 190 electrically coupled to the platen 134. The plasma doping system may further include a controller 156 and a user interface system 158. The controller 156 can be or include a general-purpose computer or network of general-purpose computers that may be programmed to perform desired input/output functions. The controller 156 may also include communication devices, data storage devices, and software. The user interface system 158 may include devices such as touch screens, keyboards, user pointing devices, displays, printers, etc. to allow a user to input commands and/or data and/or to monitor the plasma doping apparatus via the controller 156. A shield ring 194 may be disposed around the platen 134 to improve the uniformity of implanted ion distribution near the edge of the workpiece 138. One or more Faraday sensors such as Faraday cup 199 may also be positioned in the shield ring 194 to sense ion beam current.
In operation, the gas source 104 supplies a primary dopant gas containing a desired dopant for implantation into the workpiece 138. The source 101 is configured to generate the plasma 140 within the process chamber 102. The source 101 may be controlled by the controller 156. To generate the plasma 140, the RF source 150 resonates RF currents in at least one of the RF antennas 126, 146 to produce an oscillating magnetic field. The oscillating magnetic field induces RF currents into the process chamber 102. The RF currents in the process chamber 102 excite and ionize the primary dopant gas to generate the plasma 140.
The bias power supply 190 provides a pulsed platen signal having a pulse ON and OFF periods to bias the platen 134 and hence the workpiece 138 to accelerate ions 109 from the plasma 140 towards the workpiece 138. The ions 109 may be positively charged ions and hence the pulse ON periods of the pulsed platen signal may be negative voltage pulses with respect to the process chamber 102 to attract the positively charged ions. The frequency of the pulsed platen signal and/or the duty cycle of the pulses may be selected to provide a desired dose rate. The amplitude of the pulsed platen signal may be selected to provide a desired energy.
A drawback with conventional plasma processing is the lack of angular spread control of the ions 109. As structures on the workpiece become smaller and as three dimensional structures on the surface of the workpiece become more common (e.g., trench capacitors, vertical channel transistors such as FinFETs), it would be beneficial to have greater angle control. For example, FIG. 2 shows a FinFET 200 having an exaggerated size for clarity of illustration. Regions 201 are active regions, which must be implanted with ions. Using conventional plasma processing systems, it is difficult to implant ions in surfaces that are not parallel to the platen and workpiece surface. One possible approach to overcoming this is by tilting the platen. However, if the FinFET (or other feature) has a high aspect ratio (defined as the ratio of its height to its width), the maximum tilt angle is limited. For example, FIG. 2B shows a set of features 200, each having a height h and spaced apart by a width w. In order for ions to reach the lowest corner 211 of the feature, the platen must have a tilt angle (θc) no greater than that given by the equation,
                    tan                  -          1                    ⁡              (                  θ          c                )              =          w      h        ,where w is the width between features and h is the height of the feature. Thus, as the aspect ratio increases, the maximum tilt angle decreases, making it difficult to implant an adequate amount of ions in the regions 201.
In typical plasmas, sheaths form at the plasma boundaries. These plasmas typically comprise positive ions and free electrons. These two charged species have vastly different masses. Referring to FIG. 3, the presence of this sheath causes equipotential field lines 300 to be parallel to the workpiece surface 310. The formation and shape of the plasma causes the plasma to be at a voltage Vp within the sheath. This voltage is typically positive, as the electrons are separated from the positive ions. The platen, and therefore the workpiece surface 310 are held at a different voltage, such as Vcathode. To transition between these two voltages, a potential gradient exists. Due to the relative shape of the plasma and workpiece surface, this gradient can be represented by parallel equipotential lines 300. It is known that ions 109 tend to travel in paths that are perpendicular to the equipotential field lines as they are accelerated toward to the platen and workpiece surface. Thus, in this case, the positive ions 109 strike the workpiece surface orthogonally. While this is acceptable for traditional workpiece processing, it is ineffective in processing three-dimensional features, such as FinFETs. Thus, existing techniques may be inadequate for conformal processing.
Accordingly, there is a need for a plasma processing method that overcomes the above-described inadequacies and shortcomings.